The invention relates to an arrangement for supplying a medium to an exhaust line of a combustion engine and for vaporizing the medium in the exhaust line.
A technique known as SCR (selective catalytic reduction) is used for reducing discharges of nitrogen oxides from diesel engines. This technique involves adding a solution of urea in a specified dose to the exhaust gases in the exhaust line of a diesel engine. The urea solution may be sprayed into the exhaust line in such a way that it is finely divided, after which the urea solution in contact with the hot exhaust gases vaporises and ammonia is formed. The mixture of ammonia and exhaust gases is led thereafter through a catalyst in which a chemical reaction takes place. The nitrogen of the nitrogen oxides in the exhaust gases reacts here with the nitrogen in the ammonia, with the result that nitrogen gas is formed. The oxygen of the nitrogen oxides reacts with the hydrogen in the ammonia, with the result that water is formed. The nitrogen oxides in the exhaust gases are thus reduced in the catalyst to nitrogen gas and water vapour. Correct dosage of urea enables the diesel engine's discharge of nitrogen oxides to be to a great extent reduced.
During large parts of the operating range of a diesel engine, the exhaust gases are at a sufficiently high temperature to vaporise the urea solution so that ammonia is formed. It is difficult, however, to prevent some of the urea supplied from coming into contact with and becoming attached to the inside wall surface of the exhaust line in an unvaporised state. The exhaust line of a vehicle is usually in contact with ambient air. The exhaust line is cooled by ambient air to a lower temperature than that of the exhaust gases inside the exhaust line. In most cases, however, the exhaust line is at a sufficiently high temperature to vaporise and convert to ammonia the urea solution which comes into contact with the inside wall surface of the exhaust line. In cases where the exhaust line is at too low a temperature to vaporise and convert the urea solution to ammonia, urea solution will accumulate on the inside wall surface of the exhaust line. The urea solution may crystallise on the inside of the exhaust line or leak out via joints of the exhaust line and crystallise on the outside of the exhaust line. This may lead to malfunctions.